In recent years torsional hinged high frequency mirrors (and especially resonant high frequency mirrors) have made significant inroads as a replacement for spinning polygon mirrors as the drive engine for laser printers. These torsional hinged high speed resonant mirrors are less expensive and require less energy or drive power than the earlier polygon mirrors.
As a result of the observed advantages of using the torsional hinged mirrors in high speed printers, interest has developed concerning the possibility of also using a similar mirror system for video displays that are generated by scan lines on a display surface.
Standard CRT (cathode ray tube) video systems for displaying scan-lines use a low frequency positioning circuit, which synchronizes the display frame rate with an incoming video signal, and a high frequency drive circuit, which generates the individual image lines (scan lines) of the video. In the prior art systems, the high speed circuit operates at a frequency that is an exact even multiple of the frequency of the low speed drive signal and this relationship simplifies the task of synchronization. Therefore, it would appear that a very simple corresponding torsional hinged mirror system would use a high speed torsional hinged scanning mirror to generate scan lines and a slower torsional hinged mirror to provide the orthogonal motion necessary to position or space the scan lines and thereby produce a raster “scan” similar to the raster scan of the electron beam of a CRT. Unfortunately, the problem is more complex than that. The scanning oscillations of a high speed resonant scanning mirror cannot simply be selected to have a frequency that is an exact even multiple of the positioning motion of the low frequency mirror.
More specifically, the orthogonal positioning motion and, consequently, the low frequency mirror drive signal must be tied to the incoming image frame rate of the video signals to avoid noticeable jumps or jitter in the display. At the same time, however, the high frequency mirror must run or oscillate at substantially its resonant frequency, since driving a high-Q mirror at a frequency only slightly different than the resonant frequency will result in a significant decrease in the amplitude of the beam sweep (i.e. reduce the beam envelope). This would cause a significant and unacceptable compression of the image on the display. Therefore, the high speed mirror drive is decoupled from the low speed mirror drive. That is, as mentioned above, the high speed drive signal cannot simply be selected to be an even multiple of the low speed drive signal.
However, in a video display, each frame of incoming video signals representing video pixels (such as might be received from a DVD player or a VCR player) must still be faithfully reproduced. This means, each pixel of each successive frame of video must be properly located on the screen of the display in both the vertical and horizontal directions if distortions are to be avoided. At the same time, however, and as described above in a torsional hinged mirror based video system, the low frequency mirror drive must still be synchronized to the flow rate of the incoming video signals, and the high speed mirror must oscillate at its resonant frequency. Therefore, since the resonant frequency of the high speed mirror may vary from unit to unit because of manufacturing variables or with environmental conditions, the high speed mirror must be decoupled from the low speed or orthogonal moving mirror. At the same time, the individual pixels of an image must be properly located in both the vertical and horizontal direction to assure high quality images. These problems are even further complicated if an image is generated in both directions by the scanning mirror.
Therefore, a mirror based video system that can meet these requirements would be advantageous.